The present invention relates generally to a protective relay system and particularly to a method and system for identifying the direction of a fault in an electric power system, which are particularly advantageous in a close-in fault, i.e., where a fault occurs very close to the observation point.
Prior art direction-distance protective relays are formed to respond to a specified relationship between the vectors of a voltage and a current. In other words, they, in principle, resort to the fundamental wave of the system frequency. In a close-in fault, where the voltage becomes too small to serve as a basis for direction identification, so-called stored voltage data are used. But, here again, only the fundamental wave is used. Recently, however, wave distortion which occurs upon fault in an electric power system is no longer negligible, and now it is not necessarily right to identify the direction from the phase relationship between stored pre-fault voltage data and fault current data (i.e., the data of the current during the fault). This is because the phase relationship varies, in its appearance, from one instant to the next.
A method disclosed in a literature "IEEE paper No. F77052-4, Jan. 4, '77, W. D. Breingan et al." uses an algorithm which holds not only with fundamental waves but also with distorted waves. This method calculates a fictitious value of inductance from the stored voltage data, and the resultant fictitious value represents the inductance behind the observation point. If this value is positive, it is judged that the fault point is in front of the observation point as assumed. If the value is negative, it is judged that the fault point is behind the observation point, contrary to the assumption.
In the method described, it is presupposed that inductance is dominant behind the observation point. But the distorted waves which are now problematical are those due to free oscillation which occurs because of existence of capacitance of cables and shunt capacitors, in addition to inductance, behind the observation point. Therefore, the presupposition itself does not hold, and the problem is not yet solved.